Defective bone remodeling is the pathophysiologic basis of most metabolic bone diseases, including postmenopausal and age-dependent osteoporosis. In previous work, we have found that osteoblasts express members of the cadherin superfamily of cell adhesion molecules, in particular cadherin-11 (Cad11) and N-cadherin (Ncad). Cadherins mediate cell-cell adhesion, but they also intersect the Wnt signaling pathway by stabilizing -catenin on the cell surface. Cell-cell adhesion is also a pre-requisite for assembly of gap junctions and intercellular communication. We have demonstrated that either dominant-negative disruption of cadherin function or recessive null mutations of the Ncad and/or Cad11 genes (Cdh2 and Cadh11) in mice hinders bone formation, leading to low peak bone mass and osteopenia. We also find that cadherin deficiency negatively affects the Wnt/-catenin system and reduces the abundance of intercellular junctions (adherens and gap junctions), in vitro. The central hypothesis of this project is that cadherins (Ncad and Cad11) control osteogenic differentiation by modulating cell-cell interactions in the bone marrow microenvironment, via cell-cell adhesion, communication and signaling. We further hypothesize that Ncad and Cad11 have partially overlapping, yet distinct roles in the osteoblast differentiation program. To achieve this goal, we propose to determine, 1) the relative roles of Cdh2 and Cdh11 in bone forming cell commitment and differentiation and proliferation in the post-natal skeleton; 2) the interactions between cadherins (Ncad and Cad11) and Wnt signaling in osteoblast differentiation and function and 3) cadherin dependent organization and function of intercellular junctions (adherens and gap junctions) in osteogenic differentiation. We will use multiple in vitro, ex vivo and in vivo approaches, based on cadherin gene ablation mouse models we have developed, to study the consequences of cadherin deficiency on bone development, bone mass and osteoprogenitor cell recruitment and differentiation. We will also determine the cellular and molecular bases of the osteopenia of cadherin deficient mice. This proposal addresses fundamental mechanisms by which bone turnover is modulated in the bone microenvironment. Understanding the role of cadherins in bone biology is essential to gain a full picture of the molecular network by which bone development and homeostasis are controlled.